Alluvial fan river incision during climatic change: new clue from an experimental erosion device

At the foot of a mountain range, rivers sometimes cut several hundred meters into alluvial fans. This incision reflects a transition from aggradation to erosion, possibly as a result from a major change in external conditions. The incision of rivers into alluvial fans has often been interpreted as the result of a deficit in sediment supply during the transition from glacial to interglacial periods, associated with an increase in water discharge. However, such incision is not observed along all mountain fronts, nor at the mouth of all the catchment areas along a same mountain range. The conditions that lead to incision of alluvial fan are still poorly  quantified.

We tackle this question by means of a laboratory-scale device in which a 40x60cm block is uplifted at a constant rate under artificial precipitation, forming a mountainous landscape. Additionally, the products of its erosion are deposited on a plateau surrounding the block, forming alluvial fans and a piedmont. We carried out 8 experiments (700 to 900 min-long) in which the rate of precipitation was either constant or alternated between periods of low precipitation P1 and high precipitation P2, of variable duration from 10 to 40 minutes. The topography of experiments was monitored over time by using a high-resolution laser sheet. Sediment and water fluxes Qs and Qw at catchments’ outlet were computed over time from topographic and rainfall data.

We observe a variability in the relationships between Qs and Qw between catchments, due to some spatial heterogeneities in precipitation rate and migration of ridgelines. Qs and Qw vary in phase with precipitation cycles, but with different amplitudes between catchments. When the system reaches a state of dynamic equilibrium, the piedmont is a bypass for the sediments. Its average slope is inversely proportional to the average rainfall rate.

Only 8 deep incision events occurred at the outlet of certain catchments in the piedmont in all our experiments. We show that these incisions only occur for a certain slope threshold during dynamic equilibrium, and for a certain percentage decrease in sediment concentration d(Qs/Qw). These incisions never occurred in the two constant precipitation experiments and only initiated during P2 precipitation periods.

On the basis of the slope of the alluvial fans S at dynamic equilibrium, we calibrated a sediment transport law for the piedmont in the form of an excess shear stress power law. Using this law, we used a Monte Carlo approach to simulate many pairs (S , d(Qs/Qw)), for which we calculated the ratio between sediment transport capacity and flux, and the Froude number. We show that the incisions occurring in our experiments correspond to a transition towards an excess of transport capacity, for Froude numbers approaching 1. It has recently been shown that a river bed becomes unstable for such values, driving incision and knickpoints. Our experiments and theoretical analysis are consistent with this interpretation, which explains why incision occurs rarely in our experiments. These results offer new perspectives for quantitatively interpreting the incisions observed in nature in terms of paleo-fluxes Qs and Qw.